Originally Posted by
pierce
the auto industry liked to call aluminum alloy wheels "mags" because many of them contained some small percentage of magnesium, but in fact virtually all of them are mostly aluminum.
The first light weight cast wheels were indeed magnesium alloy (the metal was mostly magnesium with small amounts of alloying elements) due to it's light weight but the high cost made them unattractive for wider use so cast or forged aluminum alloy replace magnesium as the metal of choice. However, the name "Mag" stuck even when applied to wheels made of other metals.